1. Field of the Invention
The present invention relates to an image formation apparatus which contains a one-dimensional light emission element array for recording, and more particularly to an image formation apparatus of which image formation speed can be changed into the speed previously related to an image formation mode.
2. Related Background Art
A self-scanning type LED array (called a SLED hereinafter) has been introduced in Japanese Patent Application (Laid Open) Nos. 1-238962, 2-208067, 2-212170, 3-194978, 4-5872, 4-23367, 4-296579 and 5-84971, Japanese Utility Model No. 3-20457, Japan Hard-copy Memoir 1991 (A-17) xe2x80x9cProposal of Light Emission Element Array for Light Printer in which Driving Circuits Have Been Integratedxe2x80x9d, IEICE (Institute of Electronics, Information and Communication Engineers) Memoir (Mar. 5, 1990) xe2x80x9cProposal of Self-Scanning Type Light Emission Element (SLED) Using PNPN Thyristor Structurexe2x80x9d, and the like, and has been paid attention to as a light emission element for recording.
Next, an operation in a case where all of light emission elements in a circuit shown in FIG. 8 are lit will be explained with reference to timing charts shown in FIGS. 9A to 9I. When the levels of shift pulses xcfx861 and xcfx862 are 5V respectively, and when the level of a start pulse xcfx86S is changed from 0V to 5V in the state that the level of image data xcfx86I is 5V, a voltage Va of a node a becomes Va=5V, a voltage Vb of a node b becomes Vb=3.7V (it is assumed that forward voltage drop of a diode is 1.3V), a voltage Vc of a node c becomes Vc=2.4V, a voltage Vd of a node d becomes Vd=1.1V, and a voltage Ve of a node e and voltages of following nodes become Ve=0V (FIGS. 9E to 9I), whereby the level of the gate signal of a thyristor for transfer (simply referred as a transfer thyristor hereinafter) S1xe2x80x2 is changed from 0V to 5V and the level of the gate signal of a transfer thyristor S2xe2x80x2 is changed from 0V to 3.7V.
In this state, when the level of the shift pulse xcfx861 is changed from 5V to 0V (FIG. 9B), the anode, cathode and gate voltages of the transfer thyristor S1xe2x80x2 become 5V, 0V and 3.7V respectively, thereby satisfying an ON condition of the transfer thyristor. Thus, the transfer thyristor S1xe2x80x2 is turned on. Even if the level of the start pulse xcfx86S is changed to 0V in this state (FIG. 9A), the voltage Va of the node a becomes Va ≅ 5V (FIG. 9E). This is because the start pulse xcfx86S is supplied through a resistor, and a voltage between the anode and a gate of the thyristor becomes substantially identical when the thyristor is turned on. Thus, even if the level of the start pulse xcfx86S is changed to 0V, the ON condition of the first thyristor is maintained, and a first shift operation ends.
In this state, when the level of the shift pulse xcfx861 for the light emission thyristor is changed from 5V to 0V, the condition the same as the ON condition of the transfer thyristor is satisfied, whereby a light emission thyristor S1 is turned on, and the first light emission thyristor is lit. In the first thyristor, when the level of the shift pulse xcfx861 is returned to 5V (FIG. 9B), a voltage difference between the anode and cathode of the light emission thyristor becomes zero, and thus a minimum holding current of the thyristor cannot flow, whereby the light emission thyristor S1 is turned off.
Next, transfer of the ON condition of the thyristor from the transfer thyristor S1xe2x80x2 to the transfer thyristor S2xe2x80x2 will be explained. Since the level of the shift pulse xcfx861 is maintained to 0V even if the light emission thyristor S1 is turned off, the transfer thyristor S1xe2x80x2 is still on, and the gate voltage (i.e., the voltage of the node a) of the transfer thyristor S1xe2x80x2 satisfies Va≅5V. Further, the voltage Vb of the node b satisfies Vb=3.7V.
In this state, when the level of the shift pulse xcfx862 is changed from 5V to 0V (FIG. 9C), the anode voltage of the transfer thyristor S2xe2x80x2 becomes 5V, the cathode voltage thereof becomes 0V, and the gate voltage thereof becomes 3.7V (FIG. 9F), whereby the transfer thyristor S2xe2x80x2 is turned on. After the transfer thyristor S2xe2x80x2 has been turned on, when the level of the shift pulse xcfx861 is changed from 0V to 5V (FIG. 9B), the transfer thyristor S1xe2x80x2 is turned off as well as the light emission thyristor S1 being turned off. Thus, the ON condition of the transfer thyristor is shifted from the thyristor S1xe2x80x2 to the thyristor S2xe2x80x2. Then, when the level of the shift pulse xcfx861 is changed from 5V to 0V (FIG. 9B), a light emission thyristor S2 is turned on and lit.
The reason why only the light emission thyristor corresponding to the transfer thyristor which is on can perform light emission is as follows. Namely, when the transfer thyristor is not on, since the gate voltages of the thyristors except for the thyristor adjacent to the thyristor which is on are 0V, the ON condition of the thyristor is not satisfied. With respect to the adjacent thyristor, when the light emission thyristor is turned on, the voltage level of the shift pulse xcfx861 becomes 3.4V (corresponding to forward voltage drop of the light emission thyristor). Thus, since a voltage difference between the gate and cathode of the adjacent thyristor is zero, this thyristor cannot be turned on.
An SLED head in which SLED chips each having such the circuit structure as shown in FIG. 8 are arranged in array can output a light quantity necessary to expose a photosensitive body of the image formation apparatus, by the light emission of the light emission thyristor.
In a conventional printer which uses the SLED head, when printer speed is changed, also SLED driving speed is changed according to the printer speed. Conversely, although the SLED driving speed is not changed, a driving operation for the transfer thyristor in the SLED is not stopped but continuously driven even in a printing line to which any exposure is not necessary.
However, although the transfer thyristor which sequentially shifts light emission bits with respect to the light emission thyristor has the same structure as that of the light emission thyristor, a light emission quantity of the transfer thyristor is smaller than that of the light emission thyristor. For this reason, when this SLED head is used as the exposure source of the printer, there is a problem that slight light quantity unevenness occurs due to the light emission of the transfer thyristor.
Especially, in a case where printing speed of the printer is variable and rotating speed of a photosensitive drum is lowered, if transfer of the SLED head is delayed and also driving of the light emission thyristor is delayed, a duty of exposure for one line of the photosensitive drum increases, whereby a light emission time of the transfer thyristor is prolonged. For this reason, there is a problem that light quantity unevenness occurs due to the light emission of the transfer thyristor, and image unevenness resultingly thickens and becomes hard.
Further, in a case where the printer speed is adjusted to 1/2n, when the driving is performed without changing the SLED transfer speed and the light emission (exposure) speed, the SLED is kept driven without stopping the above transfer operation for each line. Thus, the transfer thyristor performs the light emission even when the exposure by the light emission thyristor is not performed, whereby there is a problem that image unevenness occurs due to a light emission quantity of a 2n-multiple transfer unit.
An object of the present invention is to provide an image formation apparatus which can solve the above-described problems and reduce image unevenness due to light emission of a transfer element.
In order to achieve the above object, the present invention is characterized by providing an image formation apparatus which can change process speed, the apparatus comprising:
a recording element array having plural recording elements to perform recording on a recording medium;
scanning means for scanning the recording element array;
control means for performing control in a low process speed mode such that a non-recording time during which no image is recorded is inserted between a recording time during which image data of one line is recorded and a recording time during which image data of next one line is recorded; and
inhibition means for inhibiting the scanning means from scanning the recording element array in the non-recording time of the low process speed mode.
Further, the present invention is characterized by providing an image formation method in which process speed can be changed and a recording element array is used, the method comprising the steps of:
in a low process speed mode, providing a recording time during which an image of one line is recorded, and a non-recording time, during which no image is recorded, between the recording time and a recording time during which an image of next one line is recorded;
scanning the recording element array to drive a recording element on the basis of image data in the recording time; and
inhibiting the scanning to the recording element in the non-recording time.
Other objects and features of the present invention will become apparent from the following detailed description and the attached drawings.